Variable capacitors have long been used in tuning circuits in connection with a fixed inductor for tuning a time constant. Originally, the variable capacitance was provided using an electromechanical device. For example, a rotary knob could be manually rotated to vary capacitance in the tuning circuit. More recently, electrically controlled variable capacitors, also known as varactors, have found widespread acceptance.
One type of varactor is a varactor diode which uses a barrier voltage at a reverse-bias junction to enable the junction to serve as a capacitance because of the separated charges in a depletion zone. The amount of junction capacitance can be electrically controlled using the reverse voltage. The capacitance changes as the depletion zone is made wider or narrower by changes in voltage.
Another type of variable varactor is a metal oxide silicon (MOS) type varactor. A MOS varactor uses an insulator layer over a semiconductor element with a gate electrode over the insulator layer to form the varactor. MOS varactors have a large amount of tunability over a narrow range of voltage. This results in a device which may be too sensitive for many applications. The effective tuning voltage range is restricted to approximately the silicon band gap of 1.2 volts. When noise factors are introduced, the problems are worsened.
The present invention is directed to solving one or more of the problems discussed above in a novel and simple manner.
In accordance with the invention there is provided a variable voltage capacitor having an increased tuning range.
Broadly, there is disclosed herein a varactor circuit having an increased tuning range comprising a first varactor in series with a second varactor between first and second terminals. A resistor is connected between the first and second terminals. A tap of the resistor is connected to a junction of the first and second varactors.
It is a feature of the invention that the varactors comprise MOS varactors. More particularly, the varactors comprise an N-FET in an N-well.
It is another feature of the invention that a diffusion of the first varactor is connected to a gate of the second varactor.
It is an additional feature of the invention that the first terminal is a gate of the first varactor and the second terminal is a diffusion of the second varactor.
It is still another feature of the invention that the resistor comprises a high-value resistor.
It is still a further feature of the invention that the resistor comprises first and second series connected resistors having a junction defining the tap.
There is disclosed in accordance with another aspect of the invention a MOS varactor device having increased tuning range and comprising a substrate. A first varactor element on the substrate has a gate and a diffusion. A second varactor element on the substrate has a gate and a diffusion. The first varactor element is connected in series with the second varactor element between first and second terminals. A resistor element is connected between the first and second terminals. A tap of the resistor element is connected to a junction of the first and second varactor elements.
There is disclosed in accordance with another aspect of the invention a variable capacitor having an increased tuning range including a first varactor in series with a second varactor between first and second terminals. A first resistor is in series with a second resistor connected between the first and second terminals. A junction of the first and second resistors is connected to a junction of the first and second varactors.
It is a feature of the invention that the first and second resistors have equivalent resistance.
Further features and advantages of the invention will be readily apparent from the specification and from the drawing.